Manufacturable spin and spin-polaron interconnects

ABSTRACT

Manufacturable spin and spin-polaron interconnects are disclosed that do not exhibit the same increase in resistivity shown by Cu interconnects associated with decreasing linewidth. These interconnects rely on the transmission of spin as opposed to charge. Two types of graphene based interconnect approaches are explored, one involving the injection and diffusive transport of discrete spin-polarized carriers, and the other involving coherent spin polarization of graphene charge carriers due to exchange interactions with localized substrate spins. Such devices are manufacturable as well as scalable (methods for their fabrication exist, and the interconnects are based on direct growth, rather than physical transfer or metal catalyst formation). Performance at or above 300 K, as opposed to cryogenic temperatures, is the performance criteria.

PRIORITY DATA AND INCORPORATION BY REFERENCE

This application claims benefit of priority to U.S. Provisional PatentApplication No. 61/932,288 filed Jan. 28, 2014, as well as U.S. patentapplication Ser. No. 14/188,736, which in turn claims benefit of thefiling of U.S. Pat. No. 8,748,957 filed Jan. 5, 2012, all of which areincorporated by reference in their entirety.

BACKGROUND OF THE INVENTION

Introduction

We describe herein the manufacturability and performance ofgraphene-based spin and spin-polaron interconnects as a function ofinterconnect line width (W) and length (L), at or above roomtemperature. This effort reflects a combination of surface chemistry andgrowth studies with advanced patterning and charge transportmeasurements and spin transport and polarization studies. The objectivesare threefold:

1) We demonstrate of the formation of graphene/dielectricheterostructures by manufacturable methods, including molecular beamepitaxy (MBE), atomic layer deposition (ALD), and plasma-enhancedchemical vapor deposition (PECVD). We have demonstrated the fabricationof such heterostructures previously, including graphene/BN formation[1], and graphene/Co3O4 formation [2].

2) We demonstrate the patterning of such structures for interconnectapplications, with systematically varied W and L, using advancedlithographic methods.

3) We illustrate the measurement of charge and spin transport andrelated magnetic behaviors as function of interconnect W and L, for bothdiffusive spin transport and magnetic polaron spin transport (FIG. 1).Importantly, these measurements focus on performance at or above 300 K,rather than at cryogenic temperatures.

“Interconnects” or interconnections are the lines of conductive materialprinted or deposited to provide for electrical connection betweenelectrically isolated or insulated devices or units that, onceconnected, form a “printed circuit.” Conventional SiCMOS based printedcircuits have adopted copper interconnect technology. The precipitousincrease in Cu interconnect resistivity with decreasing linewidth [3]poses a fundamental obstacle to continued complementary metal oxidesemiconductor (CMOS) technology scaling. Recent findings, however,concerning long spin diffusion lengths at 4.2 K (˜4-100 μm) in graphene[4-6] support a beyond-Cu/CMOS interconnect architecture based on thetransmission of spin, rather than charge.

“Conventional” graphene-based spintronics involves the injection anddiffusive transport of discrete spin-polarized carriers [6]. Suchdevices are, as with conventional interconnects, impacted by sidewallscattering of discrete carriers [7]. While the magnetoresistance, beinga ratio of conductances, might scale to smaller linewidths andunpolarized conductances, and exhibit lower energy usage characteristics[8] recent modeling suggests that such diffusive spin transportinterconnects would display enhanced resistance at smaller W [7].

Spin polaron transport, as shown schematically in FIG. 1, involves thecoherent spin polarization of graphene charge carriers due to exchangeinteractions with localized substrate spins (e.g., Co₃O₄) as in FIG. 1a. This results in formation of a “spin polaron” state stabilizedrelative to the system ground state by these exchange interactions (FIG.1b ). Such interactions occur in Mn⁺²-doped CdTe quantum wells [9], andhave recently been indicated in graphene/Co₃O₄/Co(0001)heterostructures, at and above 300 K [10] (FIG. 1c ). Since such spinpolarons are manifested due to interfacial exchange interactions, theyshould be relatively unaffected by sidewall scattering. Indeed,preliminary calculations indicate the maintenance of such spin polaronsto W˜50 nm, and possibly to much smaller dimensions. This is in contrastto Cu interconnects, where significant increases in resistance areobserved at or below W˜80 nm [3].

We compare herein the different responses of diffusive vs. magneticpolaron spin transport to sidewall scattering at small W and large L.Additionally, we explore the potential for novel, hybrid structurescombining traditional interconnect and switching functions, with thepotential for truly innovative spin-based architectures that differqualitatively from Cu/Si/CMOS. The potential of graphene-based spindevices for high speed/low power device applications has recently beenmodeled [8]. The ability of graphene-based structures to combine bothinterconnect/device/memory capabilities into new, disruptive hybridfunctionalities provides an additional and powerful motivation for theproposed research.

Related Art

The references cited at the end of this specification are identifiedlargely for the purposes of illustrating the problems and the structuresof the prior art. It is only in the context of such prior art that theinterconnect advance of this case can be properly understood.Nonetheless, the disclosures of these references may be helpful inunderstanding how to properly form and use the inventive subject matterdisclosed herein, and to that extent, they are incorporatedherein-by-reference.

SUMMARY OF THE INVENTION

A principle concern in the industry as it looks to alternatives forconventional SI-CMOS structures and processes is the manufacturabilityof graphene-based structures by direct growth (no physical transfer ormetal catalyst) on dielectric/metal and dielectric/ferromagneticsubstrates. Importantly, we have already demonstrated such capabilityfor graphene single or multilayer films on h-BN(0001)/Ru(0001) (FIG. 2)[1] and on Co₃O₄(111)/Co(0001) (FIG. 3) [2]. Importantly,graphene/Co₃O₄/Co heterostructures exhibit remarkable antiferromagnetichysteresis (FIG. 4) up to >400 K, demonstrating unusualgraphene-mediated magnetic properties with spintronic applications atpractical device operating temperatures [10]. We intend to demonstratethat similar properties are exhibited for othergraphene/dielectric/ferromagnetic heterostructures. We also propose andshow that such heterostructures can be fabricated into interconnectstructures, and the relationship of structure linewidth and otherfactors to the efficiency of spin polarization/transport at or above 300K. The impact of this invention will therefore be to demonstrate themanufacturability and potential performance and scalability of such spininterconnects or spin polaron interconnects at realistic operatingtemperatures.

DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated herein and constitutepart of this specification, illustrate exemplary embodiments of theinvention, and, together with the general description given above andthe detailed description given below, serve to explain the features ofthe invention.

FIG. 1 illustrates spin polaron formation. (a) Interfacial spin exchangeinteractions between graphene carriers and localized substrate spins (1)induce uniform carrier polarization—spin polarons (2); (b) The spinpolarized state is stabilized relative to the ground state byinterfacial exchange; (c) A spin valve/interconnect structure involvinggraphene/Co₃O₄/Co trilayer.

FIG. 2 reflects graphene CVD on ALD BN(0001) on Ru(0001). Low energyelectron diffraction (LEED) and STM dI/dV data for BN monolayer (ML) andgraphene/BN heterojunctions grown on Ru(0001): (a) LEED image forh-BN(0001)/Ru. Main LEED spots are bifurcated, as shown by enlarged spotimage (in red); model at right illustrates R30 (⅓×⅓) unit cell derivedfrom LEED image; (b) corresponding STM dI/dV data showing interfacialorbital hybridization (note features near +/−1 V_(g) (c) LEED image forgraphene/h-BN/Ru(0001), also with orbital hybridization; (d)corresponding STM dI/dV data [1].

FIG. 3 is LEED and corresponding line scan data for (a,b) 0.4 MLgraphene on Co₃O₄(111), and (c,d) 3 ML graphene on Co₃O₄(111). Arrows(a,c) mark diffraction spots associated with Co₃O₄(111), as do innerspots in the outer ring of bifurcated features (e.g., O1, O2—b,d). Outerspots in the outer ring of bifurcated features (e.g., G1, G2—b,d) aregraphene-related. LEED beam energy is 65 eV.

FIG. 4 reflects longitudinal magneto-optic Kerr effect (MOKE) hysteresisof graphene (three layers) on Co₃O₄(111) [1 nm]/Co(0001) [5.6 nm]trilayer measured at T=300 K (solid circles) and 400 K (open circles).Inset of the main panel shows a sketch of thegraphene/Co₃O₄(111)/Co(0001) trilayer.

FIG. 5 illustrates three approaches to spin interconnects: (a)graphene/BN/Ru (or, e.g., tungsten) with tunneling-based spininjection/diffusion; (b) graphene interconnects with tunneling-basedspin injection/diffusion, integrated on SiO₂/Si; (c) magnetic-polaroncoherent spin transport, based on polarization of graphene.FM=ferromagnet (Fe, Co, or Ni). Structures will be fabricated ofvariable length (L) and width (W) as shown in (a).

FIG. 6 illustrates in cross-section view the interconnect of thisinvention. As shown, on a substrate 1000 (which may be silicon) a film1002 of ferromagnetic material is deposited. A film 1004 of boronnitride at least one atomic layer thick overlays the film 1002. A film1006 of graphene is formed on layer 1004 of boron nitride.

DETAILED DESCRIPTION OF THE INVENTION

Various embodiments will be described in detail with reference to theaccompanying drawings. The structures illustrated in the drawings arerenderings not photomicrographs. They are intended to be illustrative,not limiting. In contrast, the data of FIGS. 2-4 is verified, and reliedon to demonstrate projected performance.

Proposed Interconnects and Related Structures

Spin diffusion vs. spin polaron transport Spin interconnect structuresare fabricated as shown in FIG. 5. These manufacturable structuresdemonstrate spin transport by two different methods. The most directapproach, as in FIG. 5a,b , involves the injection/diffusion of discretespins through graphene [4, 11-16]. The interconnects illustrate thedegree to which such discrete spin transport is affected by both spindiffusion vs. length (L, FIG. 5a )) and potential sidewall scatteringfor decreasing linewidths (W, FIG. 5a ), in comparison with existingmodels [7].

An alternative inventive approach herein to spin transport involves spinpolaron formation [9, 17]. Coherent spin polarization and transport canarise from strong RKKY-type exchange interactions between delocalizedcarriers and localized spins on metal cations, as in CdTe/Mn⁺² quantumwells [9, 17] and, apparently, in graphene/Co₃O₄/Co structures (FIG. 4)[10]. Because the basic phenomenon here is uniform graphene carrierpolarization, rather than injection/diffusion of discrete spins, suchstructures may exhibit more uniform scaling vs. L and W, without Ldependent polarization characteristic of discrete spin diffusion [6],and without sidewall-induced scattering characteristic of discrete spintransport.

(1) Spin Injection/Diffusion: Graphene/BN/Ru (or W) Heterostructures(FIG. 5a )

Graphene/BN/Ru heterostructures are readily manufacturable, based on ourexisting results using ALD to form the BN single layer [1] ormultilayers [18], on Ru, followed by graphene CVD [2]. Literatureresults [19] also suggest the viability of MBE or PVD. Tungsten affordsa CMOS-compatible alternative to Ru, with potentially useful interfacialchemical interactions and effects on graphene properties.

(2) Spin Injection/Diffusion—Integration with SiO2/Si(100) (FIG. 5b )

The demonstrated ability to grow (111)-oriented Co₃O₄ on SiO₂/SiO(100)by PECVD [20] provides a direct route towards the integration of spininterconnects and related structures with Si CMOS. This structure wouldstill operate by tunneling injection/diffusion of discrete spins,although graphene carrier/Co ion RKKY-type interactions [10] may also beobserved.

(3) Magnetic Polaron Formation/Transport with Substrate Gating. (FIG. 5c)

Graphene/BN/FM (=Co, Ni, Fe) heterojunctions should exhibit stronginterfacial orbital hybridization and charge transfer [1, 21, 22], withimportant consequences for substrate-induced BN and graphene magneticbehavior [23, 24], including uniform polarization of graphene chargecarriers and coherent magnetic polaron spin transport. Such behavior(without interfacial orbital hybridization) has been demonstrated forgraphene/Co₃O₄/Co structures [10] and likely will also be observed forgraphene on other magnetic oxides such as chromia and alumina. Suchstructures will be fabricated by established methods [2, 18, 19]. Thesedevices will rely on spin transport (magnetoresistance) behavior forlonger L and smaller W, and we here demonstrate how spin polarontransport scales in a manner fundamentally different from discrete spininjection/diffusion. Spin transport in this structure may be gated bythe ferromagnetic substrate, which would permit switching betweendifferent states with large and small magnetoresistance, leading tohybrid interconnect/device structures without analogy in Si-CMOSarchitecture.

Fabrication of these heterojunctions is accompanied by characterizationusing an array of surface science methods, including low energy electrondiffraction (LEED), Raman, core and valence band photoemission (XPS,UPS), and scanning tunneling microscopy/spectroscopy (STM/STS). Basicmagnetic behavior is also be probed by magneto-optic Kerr effect (MOKE)measurements. Interconnect structures with varying L and W (e.g., FIG.5a ) are then produced, coupled with SEM/TEM characterization, andconductivity measurements. Magnetoresistance measurements, coupled withspin-polarized photoemission/inverse photoemission studies willsimilarly be conducted.

Previous experience showing that graphene-covered heterostructures arelargely inert towards ambient exposure for periods of several weeks orlonger, makes this invention possible. Incorporation of the interconnectstructures permitting shipment of samples between fabrication facilitiesand assembly points.

Initially, as reflected in FIG. 5a and FIG. 5b —interconnects readilymanufacturable based on existing results are made, and spin transportmeasurements are taken to determine magneto-resistance scaling as afunction of interconnect length and width. Decreased conductivity withdecreasing W is expected [7].

Various graphene/BN or magnetic oxides/Co heterostructures (FIG. 5c ),including the possibility of Co substrate “gating” of interconnectperformance is explored as well. Scaling of magnetoresistance withinterconnect behavior is compared to results for structures involvingspin tunneling injection and diffusion demonstrating improvedperformance for the heterostructures of the invention.

Structures are then optimized for spin transmission at and above roomtemperature, and comparisons are made of switching, power usage,durability, and other factors, with variations in structure andinterface chemistry as predicted by previous results.

REFERENCES

[1] C. Bjelkevig, Z. Mi, J. Xiao, P. A. Dowben, L. Wang, W. Mei, J. A.Kelber. Electronic structure of a graphene/hexagonal-BN heterostructuregrown on Ru(0001) by chemical vapor deposition and atomic layerdeposition: Extrinsically doped graphene. J. Phys.: Cond. Matt. 22(2010) 302002.

[2] M. Zhou, F. L. Pasquale, P. A. Dowben, A. Boosalis, M. Schubert, V.Darakchieva, R. Yakimova, J. A. Kelber. Direct graphene growth onCo3O4(111) by molecular beam epitaxy. J Phys.: Cond. Matt. (2012)072201.

[3] R. H. Dennard, J. Cai, A. Kumar. A perspective on today's scalingchallenges and possible future directions. Sol. St. Electron. 51 (2007)518-525.

[4] B. Dlubak, M. Martin, C. Deranlot, B. Servet, S. Xavier, R. Mattana,M. Sprinkle, C. Berger, W. A. DeHeer, F. Petroff, A. Anane, P. Seneor,A. Fert. Highly efficient spin transport in epitaxial graphene on SiC.Nat. Phys. 8 (2012) 557-561.

[5] M. H. Guimaraes, A. Veligura, P. J. Zomer, T. Massen, I. J.Vera-Marun, N. Tombros, B. J. van Wees. Spin transport in high qualitysuspended graphene devices. Nano Lett. 12 (2012) 35123517.

[6] N. Tombros, C. Jozsa, M. Popinciuc, H. T. Jonkman, B. J. van Wees.Electronic spin transport and spin precession in single graphene layersat room temperature. Nature. 448 (2007) 571574.

[7] S. Rakheja, A. Naeemi. Graphene nanoribbon spin interconnects fornonlocal spin-torque circuits: comparison of performance and energy perbit with CMOS circuits. IEEE Trans. Elect. Dev. 59 (2012) 51-59.

[8] D. Nikonov and I. A. Young. Overview of Beyond-CMOS Devices and aUniform Methodology for their Benchmarking, Proc. IEDM (2013) in press.

[9] D. R. Yakovlev, W. Ossau, G. Landwehr, R. N. Bicknell-Tassius, A.Waag, S. Schmeusser. Two dimensional exciton magnetic polaron inCdTe/Cd1-_(x)Mn_(x)Te. Sol. St. Commun. 82 (1992) 29-32.

[10] Y. Wang, L. Kong, F. L. Pasquale, Y. Cao, B. Dong, I. Tanabe, C.Binek, P. A. Dowben, J. A. Kelber. Graphene mediated domain formation inexchange coupled graphene/Co3O4(111)/Co(0001) trilayers. J Phys: CondMatt. 25 (2013) 472203.

[11] B. Dlubak, M.-. Martin, C. Deranlot, K. Bouzehouane, S. Fusil, R.Mattana, F. Petroff, A. Anane, P. Seneor, A. Fert. Homogeneous pinholefree 1 nm Al2O3 tunnel barriers on graphene. Appl. Phys. Lett. 101(2012) 203104.

[12] B. Dlubak, P. Seneor, A. Anane, C. Barraud, C. Deranlot, D.Deneuve, B. Servet, R. Mattana, F. Petroff, A. Fert. Are Al2O3 and MgOtunnel barriers suitable for spin injection in graphene? Appl. Phys.Lett. 97 (2010) 092502.

[13] W. H. Wang, W. Han, K. Pi, K. M. McCreary, F. Miao, W. Bao, C. N.Lau, R. K. Kawakami. Growth of atomically smooth MgO films on grapheneby molecular beam epitaxy. Appl. Phys. Lett. 93 (2008) 183107.

[14] W. Han, K. Pi, H. Wang, M. McCreary, Y. Li, W. Bao, P. Wei, J. Shi,C. N. Laun, R. K. Kawakami. Spin transport in graphite and graphene spinvalves. Proc. SPIE. 7398 (2009) 739819-1.

[15] W. Han, K. Pi, K. M. McCreary, Y. Li, J. Wong, A. G. Swartz, R. K.Kawakami. Tunneling spin injection into single layer graphene. Phys.Rev. Lett. 105 (2010) 167202.

[16] W. Han, K. M. McCreary, K. Pi, W. H. Wang, Y. Li, H. Wen, J. R.Chen, R. K. Kawakami. Spin transport and relaxation in graphene. J. Mag.and Magnet. Mater. 324 (2012) 369-381.

[17] I. A. Merkulov, D. R. Yakovlev, A. Keller, W. Ossau, J. Geurts, A.Waag, G. Landwehr, G. Karczewski, T. Wojtowicz, J. Kossut. Kineticexchange between the conduction band electrons and magnetic ions inquantum-confined structures. Phys. Rev. Lett. 83 (1999) 1431-1433.

[18] J. A. Kelber. “Direct graphene growth on dielectric substrates” inGraphene, Carbon Nanotubes and Nanostructures: Techniques andApplications, J. E. Morris and K. Iniewski (CRC Press, Boca Raton, Fla.2013) 89-113.

[19] T. Lin, Y. Wang, H. Bi, D. Wan, F. Huang, X. Xie, M. Jiang.Hydrogen flame synthesis of few-layer graphene from a solid carbonsource on hexagonal boron nitride. J. Mater. Chem. 22 (2012) 2859-2862.

[20] M. E. Donders. H. C. M. Knoops, M. C. M. van de Sanden, W. M. M.Kessels, P. H. L. Notten. Remote plasma atomic layer deposition of_(Co3O4) films. J. Electrochem. Soc. 158 (2011) G92-G96.

[21] A. B. Preobrajenski, A. S. Vinogradov, N. Martensson. Monolayer ofh-BN chemisorbed on Cu(111) and Ni(111): The role of the transitionmetal 3d state. Surf. Sci. 582 (2005) 21-30.

[22] M. Weser, Y. Rehder, K. Horn, M. Sicot, M. Fonin, A. B.Preobrajenski, E. N. Voloshina, E. Goering, Y. S. Dedkov. Inducedmagnetism of cabon atoms at the graphene/Ni(111) interface. Appl. Phys.Lett. 96 (2010) 012504.

[23] V. M. Karpan, P. A. Khomyakov, G. Giovannetti, A. A. Starikov, P.J. Kelly. Ni(111/graphene/h-BN junctions as ideal spin injectors. Phys.Rev. B 84 (2011) 153406.

[24] O. V. Yazev, A. Pasquarello. Magnetoresistive junctions based onepitaxial graphene and hexagonal boron nitride. Phys. Rev. B 80 (2009)035408.

What is claimed is:
 1. An interconnect for connecting two electricaltransistors of an integrated circuit that are otherwise electricallyinsulated from each other, comprising: a film of ferromagnetic materialdeposited on a substrate; a film of boron nitride of at least one atomiclayer thickness overlaying said film of ferromagnetic material; a filmof graphene overlaying said film of boron nitride; and wherein saidinterconnect extends from one of said transistors to a second of saidtransistors.
 2. The interconnect of claim 1, wherein said boron nitrideis at least two atomic layers thick.
 3. The interconnect of claim 1,wherein said ferromagnetic material is selected from the groupconsisting of ruthenium, cobalt, nickel, iron and mixtures and alloysthereof.
 4. The interconnect of claim 3, wherein said ferromagneticmaterial comprises cobalt.
 5. An interconnect for connecting twoelectrical transistors of an integrated circuit formed on a silicon basethat are otherwise electrically insulated from each other, saidinterconnect comprising: a film of ferromagnetic material formed on saidbase; a film of a metal oxide overlaying said ferromagnetic material; afilm of graphene overlaying said metal oxide; and wherein saidinterconnect extends from one of said transistors to a second of saidtransistors.
 6. The interconnect of claim 5, wherein said interconnectfurther comprises a layer of boron nitride between said magnetic oxideand said graphene.
 7. The interconnect of claim 5, wherein saidferromagnetic material is cobalt, and said magnetic oxide is chromia. 8.The interconnect of claim 5, wherein said ferromagnetic material iscobalt, and said magnetic oxide is cobalt oxide.